This invention relates to a flat cell, particularly to a sealing member for a flat cell.
In recent years, according to the development of electronics, a variety of appliances such as desk-top calculators and watches has been designed toward miniaturizing and thinning, and also with regard to cells as power sources for driving these appliances, it has been required to miniaturize and thin them more and more.
In a conventional cell, however, a folded portion is provided along the peripheral edge portion of a sealing plate, and crimp sealing is carried out by use of an insulating packing, which fact makes the sealing structure intricate. For this reason, a thickness of the cell has been limited to a level of 1.2 mm, and an ultra-thin cell having a thickness of 1.0 mm or less has not been obtained.
Thus, in order to thin the cell, a new structure has been proposed in which positive and negative electrode metal plates are confronted with each other and peripheral edge portions of these plates are adhesively sealed with an adhesive sealing member, as disclosed in Japanese Unexamined Patent Publication No. 133771/1980. In this structure, the metal plates also serve as an external terminal and an internal current collector. In this suggested publication, an ionomer resin piece having strong adhesiveness to the metal plates is used as the adhesive sealing member.
The flat cell in which the ionomer resin piece is employed as the adhesive sealing member is excellent in airtight seal, and thus such a cell scarcely allows an inside electrolyte to leak therefrom during a long-term storage. For this reason, the flat cell is expected as a power source having a high reliability in point of the prolonged storage stability.
However, since containing a metallic ion, the ionomer resin piece is not good in electrical insulating properties, so that a minute electric current flows through the ionomer resin piece, i.e. a self-discharge occurs, which fact will impair the reliability of its long-term storage.